This invention relates in general to abrasives and, more particularly, to abrasives used in waterjet cutting processes.
Abrasives have been used in conventional abrasive air-jet and abrasive waterjet processes for a variety of machining and cutting applications. In abrasive air-jet processes, abrasive particles are entrained in air and are propelled through a nozzle at speeds which may be supersonic. The abrasive laden air-jet is then directed onto a substrate where the impact and shearing action of the abrasive particles causes removal of the intended surface material. While abrasive air-jets are well suited for sandblasting and deburring operations, they are impractical for precision machining because of the difficulty in controlling both the air-jet structure at supersonic speeds and the particle distribution within the jet.
In abrasive waterjet cutting, the abrasives are mixed with water or another liquid rather than air. The waterjet can generally transport greater quantities of abrasive particles and can be confined to a smaller diameter in comparison to air-jets. As a result, abrasive waterjets are capable of developing much higher energy than air-jets and can easily cut through most materials. The ability to focus and control the waterjet allows it to be used in precision machining operations to cut or otherwise machine materials, including metals such as aluminum, steel, titanium and high-nickel alloys, brittle materials such as glass, granite and marble, green and reinforced composite materials, honeycomb and sandwiched materials, and certain ceramics. Abrasive waterjets are also particularly well adapted for the shape cutting of sheets, plates and castings of these materials.
The nozzle which is used in conventional abrasive waterjet systems has a sapphire or diamond orifice which forms the high-velocity waterjet. The vacuum created by the waterjet draws abrasives into a mixing chamber which is within the nozzle and is downstream from the orifice. The abrasives then mix with the waterjet and, as a result of the transfer of momentum from the liquid, are rapidly accelerated to speeds which can be several times the speed of sound. The waterjet and entrained abrasives leave the mixing chamber and travel along an ultrahard tungsten-carbide tube which is aligned concentrically with the orifice. A focused, high-velocity stream of abrasive then exits the nozzle to perform the desired machining of the target material.
The use of abrasive waterjets to cut target materials causes little if any thermal distortion or oxidation or structural change to the cut surface. This type of cutting process thus offers significant advantages over conventional plasma or arc cutting methods. In addition, the cutting process is omnidirectional and complex contours can be easily cut in a continuous operation. Generation of airborne dust is also virtually eliminated in abrasive waterjet cutting processes. As a result, the process can be environmentally less hazardous than conventional processes which generate dust during the cutting operation.
Despite the many advantages of abrasive waterjet machining processes, the operating costs for such processes make them unsuited for many applications. Because the abrasives used in such processes contribute substantially to the operational costs, much attention has been focused on developing less costly abrasives.
Abrasives which have been used in conventional abrasive waterjet systems generally include garnet, silica sand, glass cullet, copper slag, steel shot, and olivine. Abrasives are generally selected by their material, size and shape. For example, it is generally known that waterjet cutting effectiveness increases with higher hardness of the abrasive particles relative to the hardness of the material being cut, with the relative hardness of the abrasive material having a more significant effect for hard target metals than for softer metals.
The use of ultra-hard abrasives such as aluminum oxide and silicon carbide have been attempted but they have been found to be generally impractical for use because they cause rapid abrasion of the nozzle mixing tube. Even when the mixing tube is formed from ultrahard carbide composites, the useful life of the tube can be reduced to a matter of minutes when aluminum oxide or silicon carbide is used as the abrasive.
As a result of the difficulties encountered in attempting to use ultra-hard abrasives, a less hard material such as garnet must generally be used as a waterjet abrasive to cut metals and other hard, brittle or ductile materials. Garnet has a generally acceptable hardness relative to the target metals and causes less wear on the nozzle mixing tube in comparison to aluminum oxide and silicon carbide. The use of garnet, however, adds significantly to the cost of the waterjet process because it is a relatively rare mineral and is costly to produce in a purified form.
A significant need has thus developed for a waterjet abrasive that has a cutting efficiency comparable to garnet but is less costly so that waterjet machining processes can be used more economically and for a wider range of applications.